OVERVIEW of UL 94 Flame Classifications |
UL 94, the Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing
There are two types of pre-selection test programs conducted on plastic materials to measure flammability characteristics. The first determines the material's tendency either to extinguish or to spread the flame once the specimen has been ignited. The first program is described in UL 94 which is now harmonized with IEC 60707, 60695-11-10 and 60695-11-20 and ISO 9772 and 9773.
The second test program measures the ignition resistance of the plastic to electrical ignition sources. The material's resistance to ignition and surface tracking characteristics is described in UL 746A, which is similar to the test procedures described in IEC 60112, 60695 and 60950.
UL 94 flame classifications:
As related to plastic materials offered by Professional Plastics, there are 6 flame classifications specified in UL 94 that are assigned to materials based on the results of these small-scale flame tests. These classifications listed in descending order for each of the following three groupings are used to distinguish a material's burning characteristics after test specimens have been exposed to a specified test flame under controlled laboratory conditions.
These classifications relate to materials commonly used in manufacturing enclosures, structural parts and insulators found in consumer electronic products (UL 94 5VA, 5VB, V-0, V-1, V-2, HB).
Horizontal versus Vertical Positioning
Specimens molded from the plastic material are oriented in either a horizontal or vertical position depending on the specifications of the relevant test method, and they are subjected to a defined flame ignition source for a specified period of time. In some tests, the test flame is only applied once as is the case of the horizontal burning (HB) test, while in other tests the flame is applied at least twice.
HB Flame Rating indicates that the material was tested in a Horizontal Position and found to burn at a rate less than a specified maximum.
Vertical Ratings, V2, V1 and V0 indicate that the material was tested in a Vertical Position and self-extinguished within a specified time after the ignition source was removed. The vertical ratings also indicate whether the test specimen dripped flaming particles that ignited a cotton indicator located below the sample. UL 94 also describes a method in which the test flame is applied for up to five applications in testing for a 5VA or 5VB classification. These small-scale tests measure the propensity of a material to extinguish or spread flames once it becomes ignited.
Difference in test methods and criteria
When looking at the flame ratings for plastic materials commonly molded to fabricate enclosures, structural parts and insulators found in consumer electronic products (5VA, 5VB, V-0, V-1, V-2 and HB), a material classified as 5VA or 5VB is subjected to a flame ignition source that is approximately FIVE Times More Severe than that used in the V-0, V-1, V-2 and HB tests. Also, the specimens may NOT Drip any Flaming Particles.
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FEATURES AND BENEFITS of UL 94 Flame Classifications |
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UL 94-5VA Surface Burn; Burning stops within 60 seconds, test specimens MAY NOT have a burn-through (no hole). This is the highest (most flame retardant) UL94 rating.
UL 94-5VB Surface Burn; Burning stop within 60 seconds, test specimens MAY HAVE a burn-through (A hole may be present)
UL 94 V-0 Vertical Burn; Burning stops within 10 seconds, NO flaming drips are allowed
UL 94 V-1 Vertical Burn; Burning stops within 60 seconds, NO flaming drips are allowed
UL 94 V-2 Vertical Burn; Burning stops within 60 seconds, Flaming drips ARE allowed.
UL 94 H-B Horizontal Burn; Slow horizontal burn test (H-B) are considered "self-extinguishing". This is the lowest (least flame retardant) UL94 rating.
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